Solar irradiation is a promising source of renewable energy, as the hourly incident solar flux on the surface of the earth is greater than the annual global energy consumption. This resource is utilized in a variety of applications divided mostly into two main areas: photovoltaics and photo-thermal applications. Photo-thermal applications for harvesting solar energy currently suffer from low efficiency and require high concentrations of sunlight, which add complexity and cost to the solar energy harvesting systems. These applications are divided into three categories: low, medium, and high temperature. Currently, low and medium temperature applications are limited to small-scale residential and commercial use. In high temperature applications, concentrated solar power (CSP) systems have recently been used to produce steam with the temperature of approximately 450° C. The superheated steam drives a heat engine (efficiency of 35-40%) to generate electricity. The thermal efficiency (ηth) of these systems are defined as
                              η          th                =                                            m              .                        ⁢                          h              LV                                                          C              opt                        ⁢                          q              i                                                          (        1        )            
where {dot over (m)} denotes the mass flow rate, hLV total enthalpy of liquid-vapor phase change, Copt the optical concentration, and qi the direct solar irradiation.
Solar collectors are one type of system to harvest low-temperature solar energy. In these collectors, working fluid (mostly water) flows through a solar panel to absorb irradiated power. These collectors are divided into two groups: surface and volumetric collectors. Conventional surface-based solar collectors (e.g., those which use a black surface to absorb sunlight and transfer the thermal energy to the adjacent working fluid) have a typical efficiency, ηth, of 52%. Volumetric collectors have been recently proposed and utilize nanofluids—fluids seeded with nanoparticles—to harvest solar energy. A range of nanoparticles (NPs) and fluids are considered for these collectors. For example, Otanicar et al. in “Nanofluid-based direct absorption solar collector,” Journal of Renewable and Sustainable Energy 2, 033102 (2010) used nanofluids of carbon nanotubes (CNT), graphite, and silver NPs in these solar collectors. The ηth efficiency of 55% and 57% was achieved with graphite and silver NPs, respectively. Tyagi et al. in “Predicted Efficiency of a Low-Temperature Nanofluid-Based Direct Absorption Solar Collector,” Journal of Solar Energy Engineering 131, 041004 (2009) showed 10% increase in ηth of solar collectors by using Al NPs-water as the working fluid. The enhanced efficiency of volumetric collectors compared to the surface collectors is attributed to three factors: higher absorbance of nanofluids due to NPs, uniform temperature in the fluid, and enhanced thermal conductivity of nanofluid. If NPs smaller than the mean free path of the bulk material are used, the absorption spectrum is typically broadened with no change in absorption peak leading to enhanced absorption efficiency. The measured absorbance of these nanofluids can reach 95% and the rest is the reflectance. However, the critical drawback of these systems is the high portion of heat loss by convection which is between 28-41% as measured by Otanicar et al., mentioned above. This limitation puts a cap on further development of volumetric collectors. In both surface and volumetric collectors, nearly half of the absorbed energy is dissipated to the surrounding medium and is converted to low-quality energy. This suggests new approaches are needed to minimize the dissipated heat in order to boost the photo-thermal efficiency of the solar collectors.
With another approach, localization of thermal energy is suggested to drive a thermally-activated phenomenon. Local heating of NPs is achieved through illumination by electromagnetic waves (e.g., generally, lasers). The local temperature rise around these NPs is used for a range of applications. For example, Sershen et al. in “Temperature-sensitive polymer-nanoshell composites for photothermally modulated drug delivery,” Journal of Biomedical Materials Research 51, 293-298 (2000) utilized the photo-thermal local heating for drug-delivery. Specifically, Au NPs in a microgel structure show enhanced temperatures with laser illumination and cause a collapse in the surrounding hydrogel matrix. The burst leads to the release of any soluble material held in the hydrogel. Lowe et al. in “Laser-induced temperature jump electrochemistry on gold nanoparticle-coated electrodes,” Journal of the American Chemical Society 125, 14258-9 (2003) locally heated the surface of gold nanoparticle-coated indium tin oxide (ITO) electrodes in an electrolyte solution. The induced temperature rise impacts the open-circuit potential of the electrode. Jones and Lyon in “Photothermal patterning of microgel/gold nanoparticle composite colloidal crystals,” Journal of the American Chemical Society 125, 460-5 (2003) introduced local photo-thermal heating for phase-change of a microgel/gold nanoparticle composite colloid. By illuminating with a laser resonant with Au plasmon absorption, they introduced local heating to cause a crystalline-amorphous phase change in the microgel matrix. In these approaches, high-quality energy is introduced locally to drive a physical or chemical phenomenon.
For surface plasmon (SP) induced heating, Govorov et al. developed a model to predict maximum temperature rise at the surface of plasmonic NPs as
                    T        =                              T            ∞                    +                                                    R                NP                2                                            3                ⁢                                                                  ⁢                k                                      ⁢                          ω                              8                ⁢                π                                      ⁢                                          Re                ⁡                                  (                                                            3                      ⁢                                              ɛ                        0                                                                                                            2                        ⁢                                                  ɛ                          0                                                                    +                                              ɛ                        NP                                                                              )                                            2                        ⁢                          Im              ⁡                              (                                  ɛ                  NP                                )                                      ⁢                                          8                ⁢                π                ⁢                                                                  ⁢                                  I                  0                                                            c                ⁢                                                      ɛ                    0                                                                                                          (        2        )            
where T∞ denotes the temperature of the medium, RNP2 the radius of the NP, k the thermal conductivity of the medium, ω the frequency of the incident wave, ∈0 dielectric constant of the medium, ∈NP dielectric constant of the NP, I0 the intensity of the electromagnetic wave in the medium, and c the speed of light. This equation suggests that (T−T∞)∝ RNP2. Keblinski et al. in “Limits of localized heating by electromagnetically excited nanoparticles,” Journal of Applied Physics 100, 054305 (2006) studied the limit of the global temperature rise of a NP under illumination by electromagnetic waves. They solved the diffusive heat equation with constant heat flux at the surface of the NP. In the liquid and amorphous phases due to lack of crystallinity, the mean free path is on the order of atomic distances and consequently the applicability of the diffusive heat equation at the nanoscale is justified. The maximum temperature rise on the NP is given by
                    T        =                              T            ∞                    +                                                    C                opt                            ⁢                              q                i                            ⁢                              σ                p                                                    4              ⁢              πκ              ⁢                                                          ⁢                              r                np                                                                        (        3        )            
where σp denotes the cross-sectional area of the NP and rnp the radius of NP. Similarly, by analogy with electrostatics, the temperature rise for an ensemble of NPs that form an agglomerate with the radius of Rag is
                    T        =                              T            ∞                    +                                                    ρ                N                            ⁢                              R                ag                            ⁢                              C                opt                            ⁢                              q                i                            ⁢                              σ                p                                                    2              ⁢                                                          ⁢              κ                                                          (        4        )            
where ρN denotes the concentration of particles per volume. For plasmonic NPs, a comparison of the temperature rise calculated from the existing theories and the measured temperature rise from experiments generally does not show a good agreement between the theory and the experiments. The discrepancy suggests that the fundamental understanding of heat generation at this scale is still unresolved. The interfacial thermal resistance, the role of hot electrons, and the mechanism of heat transfer at this scale (ballistic or diffusive) are among the open questions.
Local heating can be utilized in the heating or phase-change of water to harvest solar energy. Efficient harvesting of solar energy for steam generation is a key factor for a broad range of applications, from large-scale power generation, absorption chillers, and desalination systems to compact applications such as water purification for drinking, sterilization, and hygiene systems in remote areas where the only abundant energy source is the sun. Current methods of generating steam using solar energy rely on a surface or cavity to absorb the solar radiation, and transferring heat to the bulk liquid directly or via an intermediate carrier fluid. These methods, however, require high optical concentration and suffer from high optical loss and surface heat loss, or require vacuum to reduce convective heat loss under moderate optical concentration. The steam generated is usually in thermal equilibrium with the bulk liquid. Nanofluids have been studied as volumetric absorbers, potentially minimizing the surface energy loss by uniform temperature in the fluid and enhanced thermal conductivity of the nanofluid. Local generation of steam in a cold bulk liquid can be achieved through high concentrations or illumination of nanofluids by electromagnetic waves with high power intensity. Recently, Neumann et al. in “Solar vapor generation enabled by nanoparticles,” ACSNano 7, 42-9 (2012) and “Compact solar autoclave based on steam generation using broadband light-harvesting nanoparticles,” Proc. Natl. Acad. Sci. U.S.A. 110, 11677-81(2013) succeeded in the generation of steam in bulk water with Au NPs with the power of 103 kW/m (optical concentration, Copt, of 1000). However, the solar-thermal conversion efficiency of the approach was still only 24%. High optical concentrations limit the utilization of these approaches in stand-alone compact solar systems. Furthermore, high optical concentrations add complexity and cost to the solar-thermal conversion system.